This invention concerns contact lenses of a variety of designs, from the conventional monofocal, and, as well, contact lenses utilizing phase plate optics, such as phase plate bifocals and "tuned" Fresnel lenses arranged as annular and concentric zones. The latter type of lenses generally follow the unique designs described by Allen L. Cohen in U.S. Pat. Nos. 4,210,391; 4,338,005; and 4,340,283. The bifocal lens design of Cohen, supra, provides that the radii "r.sub.n " of the annular and concentric zones are substantially proportional to .sqroot.n (herein called a "Cohen lens design"). Other characterizations of multifocal lenses which would employ a Cohen lens design in operative embodiments are described by Freeman in U.S. Pat. Nos. 4,637,697; 4,641,934; and 4,642,112.
Contact lenses which utilize prism ballast, peri-ballast, truncation or some other stabilizing means, frequently lack stable lens orientation. Heretofore, this has been true in toric lenses, bifocal lenses and other contact lenses that are not rotationally symmetric. It is desirable to stabilize progressive add contact lenses and other aspheric contact lenses which need to remain in a well centered position on the eye.
Translation and orientation of a contact lens was assumed to result from the abutment of a truncated lens edge against the lower lid margin of an eye. However, this has been shown to have only a minimal effect by I. M. Borish and D. Perrigin in their article Observations of Bifocal Contact Lenses, Inter. Eyecare, 1 (3), 1985.
Orientation in contact lenses, assumed to result from a weight differential induced by prism and thereby causing a pendulum effect, has only a marginal effect according to A. L. Cohen in his article The Role of Gravity in Prism Ballasting, Amer. J. Opt. and Phys. Optics, May 1976.
Heretofore, regardless of the specific mechanism, all contact lenses that are stabilized by virtue of a geometrical asymmetry, incorporate prism into their lens design. Such lenses have wedge-like cross-sections. An underlying mechanism for contact lens orientation upon an eye is related to pressure effects upon wedges. This has been pointed out by A. J. Hanks in his article The Watermelon Seed Principle, Contact Lens Forum, September 1983.
When an upper eyelid closes over a contact lens with a wedge-like cross-section, pressure is asymmetrically applied as the lens is sandwiched between the eye and eyelid. This asymmetric squeezing pressure causes movements which preferentially direct the lens base first out from between the lid-eye sandwich. This "base first" orientation is achieved by a combination of translation and rotation.
A significant feature of such lenses as described in their ability to rest upon the eye in a stable orientation. While numerous advances have been made in lens design, these lenses are still far from perfected and often position incorrectly. A large part of their instability upon the eye relates to the excessive translational movements that these lenses undergo when subjected to a blinking eyelid.
It is recognized that there is a need for contact lenses that--
center well upon the eye, PA0 ride upon the eye with a stable orientation, and PA0 exhibit very little translational motion upon the eye.
This invention has the objective of accomplishing these needs by a new mechanism, other than by the base curve, diameter, and the like features of lenses, to aid in the fitting and stabilization of a contact lens.